It is well known that wave deformation around Okinawa Islands is very complicated because of reef topography. The objective of this study is to investigate the effects of reef length and wave condition on wave overtopping rate of a vertical seawall in an irregular wave field by conducting hydraulic model experiments. The experimental results showed the dimensionless wave overtopping rate decreases exponentially with an increase in the dimensionless reef length. It was revealed that the amplitude of long wave components in front of the seawall tends to increase with the dimensionless reef length R/h and the Ursell number Ur . Moreoverthe results indicated that the dimensionless wave overtopping rate is affected by not only the relative crown height of the seawall but also the amplitude of long wave components.